Field of the Invention
The present inventions relate to a measurement of reflection characteristics.
Description of the Related Art
A computer graphics (CG) based technique capable of generating an image that accurately reproduces the texture of a real object is conventionally known. To generate a CG image of an object in a virtual space that reflects the texture of an actually existing object, it is necessary to perform rendering processing based on measurement results of reflection characteristics of a target real object. A bidirectional reflectance distribution function (BRDF) is an exemplary function that can represent reflection characteristics of an opaque object (e.g., a metal). A bidirectional scattering surface reflectance distribution function (BSSRDF) is an exemplary function that can represent reflection characteristics of a semitransparent object (e.g., a human skin or a marble) into which incident light can enter and cause scattering. Each of these reflection characteristic functions is a multivariable function, which takes a long time to perform a fine measurement for respective variables (e.g., incident light direction and outgoing light direction). For example, in many cases, BRDF data of a metal surface reveals that the peak width is less than 1 [deg] and the luster is sharp, although it depends on the type and surface finishing of each metal. To measure a metal that is sharp in luster, it is necessary to perform a BRDF measurement with sufficiently reduced sampling width. If the BRDF measurement is performed at intervals of 1 [deg] for each of the incident light direction and the outgoing light direction, the number of times of measurement rises up to approximately 350,000,000. Such a large-scale measurement is unrealistic when the time actually required to finish the entire measurement is taken into consideration. Further, the amount of measurement data to be processed becomes huge.
In view of the above, a method for reducing the time required for the reflection characteristic measurement or a method for reducing a required storage area are conventionally proposed. As discussed in Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2002-500754, using a charge-coupled device (CCD) line camera is conventionally proposed to reduce the reflection characteristic measurement time because reflectance values can be simultaneously acquired in many outgoing directions. Further, as discussed in Japanese Patent Application Laid-Open No. 2007-26049, there is a conventional technique that restricts a reflection characteristic measurement target to fabric materials in such a way as to limit the incidence direction and the outgoing direction to be measured, so that the acquisition of reflection characteristics can be realized by a smaller number of times of measurement.
However, according to the technique discussed in Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2002-500754, it is unfeasible to sufficiently reduce the number of times of measurement for the thoroughly performed BRDF measurement because there is a limit to camera density of the line camera and the simultaneous measurement can be performed only along a line. Further, the technique discussed in Japanese Patent Application Laid-Open No. 2007-26049 is dissatisfactory in versatility to the material type because measurement target materials are limited in type, although the number of times of measurement or the storage area can be reduced to a certain extent.